In polishing applications such as chemical mechanical polishing, measures are taken to ensure that the surface being polished is subjected to uniform, isotropic polishing forces. The uniformity of the polishing force applied to the surface is a significant factor in determining the degree of surface uniformity that can be attained through polishing.
Thus, for example, in a chemical mechanical polishing machine with continuous belt polishing, the longitudinal motion of the belt is often supplemented by lateral and rotational motion of the wafer to ensure that every area of the wafer is subjected to uniform, isotropic polishing forces.
The force generated by friction between the wafer and the belt will, at any given instant, be exerted primarily in the direction of the belt movement across the surface of the wafer. Likewise, in other polishing configurations, a frictional force will be exerted by the polishing surface in the direction of movement of the polishing surface relative to the wafer. A retaining ring is generally used to counter this force and hold the wafer in position. The frictional force of the polishing surface impels the wafer against the retaining ring, which exerts a counterbalancing force to maintain the wafer in position.
The frictional force of the polishing surface and the reactive force exerted by the retaining ring on the wafer may be sufficient to cause the wafer to buckle. This buckling of the wafer may resemble a so-called Euler column familiar to those skilled in the art of material strain analysis. This buckling may result in uneven polishing of the wafer surface, particularly near the edge of the wafer. This problem has been observed in high-speed polishing, particularly for large-diameter, thin wafers.